Traditional cationic electrodeposition coating compositions are excellent in coating workability, and their coating films are also excellent in anti-corrosion properties; thus, the compositions have been widely used on automobile bodies, automobile parts, electrical equipment parts, and other industrial machinery, which are required to have such properties. These cationic electrodeposition coating compositions, however, have unsatisfactory throwing power for interior panel portions, compared with the throwing power for the exterior panel portions, and an electrodeposition coating film of about 10 μm is formed on interior panel portions, compared with an electrodeposition coating film of about 40 μm formed on exterior panel portions, particularly with welded portions having insufficient anti-corrosion properties. To address this drawback, there have been proposed a variety of cationic electrodeposition coating compositions satisfactory in throwing power and anti-corrosion properties.
For example, in Patent Literature 1, an electrodeposition coating composition contains a polyester resin to improve the throwing power and anti-corrosion properties. However, the composition may be unsatisfactory in long-term storage. In Patent Literature 2, a dimer acid is modified into an epoxy resin for a base resin to improve the anti-corrosion properties. However, the throwing power and anti-GA gas pinhole properties may not both be sufficiently achieved.
As used herein, the term “GA gas pinhole” refers to a hole formed as follows: when hydrogen gas generated in electrodeposition coating discharges an electrical current, the electrical energy (spark discharge) cures part of a coating film, and thereby leaves pinholes after baking. When GA gas pinholes are formed, the finished appearance and anti-corrosion properties of the coating film further deteriorate.